A multi-beam scanning optical system is provided with light sources that emit a plurality of laser beams having different wavelengths, respectively, a single deflector which deflects the plurality of laser beams simultaneously, an imaging optical system that converges the plurality of laser beams deflected by the single deflector on the surface to be scanned, and a beam detector that receives the plurality of laser beams directed to outside of the predetermined imaging area via at least one of lens elements included in the imaging optical system, a synchronizing signal being generated upon detection of each of the plurality of light beams by the beam detector. It should be noted that an optical characteristic of the imaging optical system is configured such that the laser beams directed to the predetermined imaging area are aligned in a scanning direction, while the beams directed to the beam detector are shifted in the scanning direction.